Seismic Performance of Indian RC Tall Buildings with Transfer Beam Located in Lower Seismic Zone (II)

Abstract

From last two decades, India is witnessing rapid urbanisation. Many tall buildings are coming up in tier I and tier II cities of India. To accommodate the functional needs of an occupant, such as car parking, within the same building, two different floor plans are needed. The requirement for unobstructed space for car parking and residential units is fulfilled by discontinuing the closely spaced vertical elements, such as structural walls and columns at certain floor levels. Elements supporting such discontinued elements are known as transfer elements, and buildings with such features are becoming popular in India. Transfer elements introduce stiffness irregularity in the structure, which is undesirable from the design code point of view. The recently introduced tall building code IS 16700 does not have detailed guidelines for analysing and designing such structures. Weather these structures are safe is the question that needs to be answered. With that question in mind, a detailed literature review is carried out to know the global practice of analysing and designing tall buildings with transfer elements located in lower seismic zones of respective countries. Overall, the literature reviewed supports the idea of constructing buildings with transfer elements in the lower seismic zone. However, these recommendations are supported by comprehensive experimental and numerical studies tailored to local seismic hazard and construction practices. Therefore, the motivation of this study is to contribute to the understanding of the global response of RC tall buildings with transfer beams subjected to seismic activity. Particular attention is given to the residential building with RC transfer beam (TB) located in seismic zone II. Along with this, the study also attempts to evaluate the advantages and disadvantages of attaching a podium to the buildings under focus. For this purpose, residential buildings inspired by current building stock are analysed and designed using relevant IS codes. And later, the seismic performance of these buildings is evaluated using Linear Time History Analysis (LTHA) method. Three global parameters, viz., base shear, displacement and inter-storey drift ratio (IDR), and two local parameters of PMM capacity ratio and shear demand to capacity ratio, are used as performance indicators. Assessment of these buildings revealed that the base shear values due to LTHA were observed to increase by 19-32% compared to the design base shear of buildings. However, roof displacements for all the buildings are found to be very lower. Also, displacement variation due to podium configurations were found to be negligible. The maximum IDR value did not exceed 1/5th of code limiting IDR values. Furthermore, despite the stiffness and mass irregularities near the TB level, the maximum IDR demand was found to be at the upper storeys. The PMM capacity ratio of all the columns of different buildings does not exceed 0.30, which indicates that the brittle failure of transfer columns will not be present. Similarly, the transfer column shear demand to capacity ratio reached only 0.50. The study found that the building performance was satisfactory under LTHA, even after qualifying for several irregularities, thus eliminating the need to revise the code limits for inter storey drift ratio. Additionally, it was found that residential buildings with RC transfer beams can perform well in seismic zone II when all analysis and design criteria are properly followed. The impact of podium configurations on the seismic resistance of buildings was found to be negligible. This implies that architects and structural engineers can choose whether or not to include a separation joint based solely on functional and execution requirements. Hence, the transfer beam feature can be allowed in seismic zone II and for zone III, IV and V further investigation is needed before it is recommended